Re #16480 add documentation for the thermal neutron bk2bk functions

This includes docs for ThermalNeutronBk2BkExpAlpha, ThermalNeutronBk2BkExpBeta, ThermalNeutronBk2BkExpSigma and ThermalNeutronDtoTOFFunction

docs/source/fitting/fitfunctions/ThermalNeutronBk2BkExpAlpha.rst

+.. _func-ThermalNeutronBk2BkExpAlpha:
+
+===========================
+ThermalNeutronBk2BkExpAlpha
+===========================
+
+.. index:: ThermalNeutronBk2BkExpAlpha
+
+Description
+-----------
+
+ThermalNeutronBk2BkExpAlpha is a function to calculate :math:`\alpha` of :ref:`ThermalNeutronBk2BkExpConvPVoigt <func-ThermalNeutronBk2BkExpConvPVoigt>`.
+It is defined as
+
+.. math:: \alpha(d_h)  = \left(n\alpha_0 + \alpha_1 d_h + (1-n)\alpha_0^t - \frac{\alpha_1^t}{d_h}\right)^{-1}
+
+where
+
+.. math:: n = \frac{1}{2} \mathit{erfc}(\text{Width}(t_\text{cross} - d^{-1}))
+
+and
+
+.. math:: erfc(x) = 1-erf(x) = 1-\frac{2}{\sqrt{\pi}}\int_0^xe^{-u^2}du.
+
+.. attributes::
+
+.. properties::
+
+.. categories::
+
+.. sourcelink::

docs/source/fitting/fitfunctions/ThermalNeutronBk2BkExpBeta.rst

+.. _func-ThermalNeutronBk2BkExpBeta:
+
+==========================
+ThermalNeutronBk2BkExpBeta
+==========================
+
+.. index:: ThermalNeutronBk2BkExpBeta
+
+Description
+-----------
+
+ThermalNeutronBk2BkExpBeta is a function to calculate :math:`\beta` of :ref:`ThermalNeutronBk2BkExpConvPVoigt <func-ThermalNeutronBk2BkExpConvPVoigt>`.
+It is defined as
+
+.. math:: \beta(d_h)  = \left(n\beta_0 + \beta_1 d_h + (1-n)\beta_0^t - \frac{\beta_1^t}{d_h}\right)^{-1}
+
+where
+
+.. math:: n = \frac{1}{2} \mathit{erfc}(\text{Width}(t_\text{cross} - d^{-1}))
+
+and
+
+.. math:: erfc(x) = 1-erf(x) = 1-\frac{2}{\sqrt{\pi}}\int_0^xe^{-u^2}du.
+
+.. attributes::
+
+.. properties::
+
+.. categories::
+
+.. sourcelink::

docs/source/fitting/fitfunctions/ThermalNeutronBk2BkExpSigma.rst

+.. _func-ThermalNeutronBk2BkExpSigma:
+
+===========================
+ThermalNeutronBk2BkExpSigma
+===========================
+
+.. index:: ThermalNeutronBk2BkExpSigma
+
+Description
+-----------
+
+ThermalNeutronBk2BkExpSigma is a function to calculate :math:`\sigma` of :ref:`ThermalNeutronBk2BkExpConvPVoigt <func-ThermalNeutronBk2BkExpConvPVoigt>`.
+It is defined as
+
+.. math:: \sigma(d_h) = \left(\sigma_0^2 + \sigma_1^2 + d_h^2 + \sigma_2^2 + d_h^4\right)^{0.5}
+
+.. attributes::
+
+.. properties::
+
+.. categories::
+
+.. sourcelink::

docs/source/fitting/fitfunctions/ThermalNeutronDtoTOFFunction.rst

+.. _func-ThermalNeutronDtoTOFFunction:
+
+============================
+ThermalNeutronDtoTOFFunction
+============================
+
+.. index:: ThermalNeutronDtoTOFFunction
+
+Description
+-----------
+
+ThermalNeutronDtoTOFFunction is a function to calculate TOF from d-spacing values for :ref:`ThermalNeutronBk2BkExpConvPVoigt <func-ThermalNeutronBk2BkExpConvPVoigt>`.
+It is defined as
+
+.. math:: TOF = n(zero + dtt1 \cdot dh)+(1-n)(zero_t + dtt1t\cdot dh - \frac{dtt2t}{dh})
+
+where
+
+.. math:: n = \frac{1}{2} \mathit{erfc}(\text{Width}(t_\text{cross} - d^{-1})).
+
+.. attributes::
+
+.. properties::
+
+.. categories::
+
+.. sourcelink::